A supplementary speed change mechanism having two fluid-operated clutches which are operated one at a time may be a direction-reversing mechanism or high/low speed-selecting mechanism, and such a supplementary speed change mechanism is set in advance before the running of vehicle in a condition where one of the fluid-operated clutches is selected for operation, so that selecting operation of the clutches is seldom performed during the running of vehicle. Contrary, a main speed change mechanism having plural fluid-operated clutches which are operated one at a time is frequently operated for shifting during the running of vehicle. In a structure in which a single hydraulic pump is used for supplying fluid to a supplementary speed change mechanism or fluid-operated clutches thereof and to a main speed change mechanism or fluid-operated clutches thereof, it is known from JP, A No. 1-199034 and from JP, A No. 8-20257 that fluid pressure applied to one of the fluid-operated clutches of supplementary speed change mechanism under operation is once reduced automatically in response to the shifting operation of main speed change mechanism so as to bring the clutch into a half-clutch condition. A fluid-operated clutch of the supplementary speed change mechanism in a half clutch condition will assure that a fluid-operated clutch of the main speed change mechanism to be newly engaged becomes operated substantially instantly without causing a shock so that a smooth speed transfer is achieved.
In the vehicle shown in JP, A No. 1-199034 wherein a direction-reversing mechanism having selectively operable two fluid-operated clutches and a main speed change mechanism having selectively operable plural fluid-operated clutches are connected in series, there is provided a pressure-unloading valve which is operable to once reduce fluid pressure in a fluid supply path connected to a directional control valve for the direction-reversing mechanism in response to the operation of directional control valve means for the main speed change mechanism. Lubrication of the two fluid-operated clutches of direction-reversing mechanism is performed by fluid relieved from a relief valve which determines fluid pressure applied to the clutches. In the vehicle shown in JP, A No. 8-20257 wherein a direction-reversing mechanism and a high/low speed-selecting mechanism each having two fluid-operated clutches are provided as supplementary speed change mechanisms and wherein these supplementary speed change mechanisms and a main speed change mechanisms having plural fluid-operated clutches are connected in series, electromagnetic proportional valves are incorporated respectively in fluid supply paths for the direction-reversing mechanism and for the high/low speed-selecting mechanism and any one of these electromagnetic proportional valves is once displaced in response to the operation of directional control valve means for the main speed change mechanism. Any reference is not made as to how the fluid-operated clutches of the direction-reversing mechanism and of the high/low speed-selecting mechanism are lubricated.
When lubricant is not supplied sufficiently to the fluid-operated clutches of a supplementary speed change mechanism which are reduced in fluid pressure applied thereto for a half clutch or slippingly engaged condition every time when the shifting operation of a main speed change mechanism is performed, seizing of the fluid-operated clutches is easily caused so that durability of the clutches is damaged.
Sufficient lubrication of the fluid-operated clutches of a supplementary speed change mechanism is also desirable in a structure wherein a selectively operable pressure-reducing valve is incorporated in a fluid supply path to the fluid-operated clutches for selectively reducing fluid pressure applied to one of the clutches under operation so as to bring the clutch into a slippingly engaged condition and to thereby slow down the running speed of vehicle.
A primary object of the present invention is to provide a novel fluid supply system which sufficiently supplies lubricant to the fluid-operated clutches of a supplementary speed change mechanism which are reduced in fluid pressure applied thereto for a half clutch or slippingly engaged condition every time when the shifting operation of a main speed change mechanism is performed.
Another object of the invention is to provide a novel fluid supply system which assures a substantially instant operation or engagement of the fluid-operated clutch to be newly engaged in a main speed change mechanism so as to attain a smooth speed transfer.
Another primary object of the present invention is to provide a novel fluid supply system which sufficiently supplies lubricant to the fluid-operated clutches of a supplementary speed change mechanism which are reduced in fluid pressure applied thereto by the selective operation of a pressure-reducing valve incorporated in a fluid supply path to the fluid-operated clutches for a selective speed-reducing purpose of the vehicle.